The invention is based on the temperature-measuring prior art, by which the depletion-layer temperature of a semiconductor rectifier device is determined directly with a temperature sensor applied to the semiconductor rectifier. However, by this method, the prevalling depletion-layer temperature can be determined only with a considerable time delay because of the distance between the depletion layer and the point of measurement. Rapid temperature changes caused by steep increases in power dissipation cannot be determined at all.
Semiconductor switches are also known in which the current to be switched, which current flows through a semiconductor rectifier, is measured and, if the current exceeds a permissible maximum value, the rectifier is opened after a predetermined time delay which delay may still be partially affected by the output voltage (see, for instance, the article by M. Raffay "UAF 1780---A Low-loss Monolithic Switch Component" in Applikationsbericht 6, SGS-Thomson Leistungs Ics, December, 1988). In these switches, however, the actual depletion-layer temperature is not measured. Such switches are generally opened too early due to the dimensioning which is necessarily adapted to the most unfavorable case of switching.